HELVETICA CHIMICAACTA- Vol. 59, Fasc. 6 (1976) - Nr. 203

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203. The Photoelectron Spectrum of Dibenzo-p-quinodimethane by Michael Allan’), Edgar Heilbronnerl) and Gerd Kauppz) Physikalisch-chemisches Institut der Universitat Basel, Klingelbergstrasse 80, CH-4056 Basel, und Chemisches Laboratorium der Universitat Freiburg i. Br., Albertstrasse 21, D-7800 Freiburg i. Br. (25. V. 76) Summary. The lowest states of dibenzo-fi-quinodimethane radical cation are 2Blu (7.95 eV) ; 2Bzg (8.66 eV) ; [2B3g,2A,, unresolved] (9.1 eV) ; [2B1,, 2Bzg, unresolved] (10.7 eV). This confirms the assignment proposed by Koenig et al. [l] for the states of 9-quinodimethane radical cation.

I n a remarkable piece of work Koenig et al. [l]recorded and analysed the photoelectron spectrum of 9-quinodimethane (I) which exhibits two bands a t 7.87 & 0.05 and 9.7 f 0.1eV with an integrated intensity ratio of 1 :1.2.These have been interpreted as being associated with the radical cation doublet states 2Blu and 2B3g, 2Bzg of 1+,respectively. (Note that the symmetry labels given in [l] differ from those used here because of an interchange in axes.) As has already been shown by Koenig et al. [l],a naive LCBO (= Linear combination of bond orbitals) model calculation accounts rather nicely for the observed band patterns). The necessary parameters (self-energies A, of the basis functions nl, and interaction terms B,, for conjugated pairs nlc,nv)can be derived as follows: 1) An investigation of cis- and trans-hexatriene (2) yielded A, = - 10.25eV and Ab = - 9.99 eV with Bab = - 1.215eV [4]. Comparing A, of 2 with A, = - 10.51 eV for ethylene we might expect that A, = - 10.0eV is appropriate for the exocyclic n-orbital in 1, whereas Ab should be the same for both molecules. 2) The analysis of the photoelectron spectra of fulvene (3) and of 3,4-dimethylenecyclobutene (4)[5]leads to A, = - 10.3 eV in 3 and 4, and to Ab = - 10.2 eV or - 10.3 eV in 3 and 4 respectively. 3) The photoelectron spectra of the hydrocarbons 5(n), i.e. 5(0) = 1,Ccyclohexadiene to 5(3) = 1,4,5,6,7,10,11,12-octahydronaphthacene [6] yielded Ab = - 10.3 eV for all n-orbitals and a ‘through-space’ interaction term [7] Bbb, = Bb,b,, = . .* = - 0.5 eV for each pair of 1,4-positioned double bonds. Using the set of parameters A, = Aa, = - 10.0 eV, Ab = Ab, = - 10.3 eV, Bab = . .. = B a f b r = - 1.215 e v , Bbb. = -0.5 eV for a LCBO description of the 1)

2)

3)

Basel. Freiburg i. Br. These authors use a somewhat different nomenclature, in which the basis functions of the linear combinations are named with reference t o a pseudo valence bond scheme [Z]. However, it can be shown that mathematically there is a complete one-to-one correspondence between the matrix elements used in their treatment and those of the more traditional LCBO-procedure

131.

1950

HELVETICA CHIMICAACTA- Vol. 59, Fasc. 6 (1976) - Nr. 203 x

t

1

2

3

i

n-system of 1 and making use of Koopmans' approximation we find the following ionization energies (in eV) : State of 1+: 'Blu

Iv,j (talc) 7.94

'B3g 'Bzg 'Blu

10.00 9.80 12.86

1

Iv,j(obs)L11 7.87 1 0 . 0 5

9.7

*

0.1

(12.5?)

The experimental value quoted for the second 2Rlu state corresponds to the position of a large prominent feature near 12.5 eV which is probably due to the superposition of two (or more) bands.

9 10 11 12 13 IONIZATION ENERGY ( e V )

8

He( I)-Photoelectron spectrum of dibenzo-p-quanodimethane

Had we used A, = Aar= 10.2 eV instead of - 10 eV, the 2Bzg state would have shifted to 10.2 eV. Such a value is still acceptable for the following reason: Assuming Koopmans' approximation, electron ejection from the LCBO-orbital bZg ~

HELVETICA CHIMICA ACTA- Vol. 59, Fasc. 6 (1976) - Nr. 203

1951

puts half a positive charge on both exocyclic double bonds of 1+, leaving the population of the endocyclic double bonds unchanged. As has been pointed out in the case of the ionization energies of fulvenes [8] such a charge distribution leads to substantial electron rearrangement and thus to a stabilization of the 2B2, state, i.e. to a reduction in the ionization energy. In support of Koenig's assignment of the photoelectron spectrum of 1 we wish to report in Fig. 1 the spectrum of dibenzo-9-quinodimethane 6 !9] obtained by pyrolysis a t approx. 200" from [2.2] (9,lO)anthracenophane7 [lo] :

7

6

Within the range from 8 to 10 eV the photoelectron spectrum of 6 consists of four bands (labelled 0 to with relativ intensities of approximately 1:1:1:1. Whereas band @ is well detached, @ overlaps partially with the double band (0, @) the latter corresponding obviously to two transitions of closely similar ionization energies. The interpretation of the spectrum is rather straightforward if one applies the LCBO scheme described above. To do so, we use as localized basis orbitals the norbitals y1 = na and y12 = na), for the exocyclic double bonds and the benzene n-orbitals ly3(6) = el,(A), y4(7)= el,(S), y5(8)= azU for the ortho-phenylene moieties to the left (right) of the xz-plane. For computational purposes all these orbitals are written in their usual Hiickel LCAO-approximation e.g. for the right side phenylene moiety "6 = e d A ) = ((52 4 3 - (511 - 412)/2, y7 = elg(S) = (241 (52 - (53 - 244 (512 (511)/1/Eand ly8 = azu = ((51 (52 (53 (54 (511 (512)/1/6where A and S refer to the antisymmetric or symmetric behaviour of the orbital relative to a plane parallel to the x,z-axes. The basis energies A3 = A4 = A6 = A7 = -9.25 eV, A5 = A8 = - 12.25 eV of the phenylene groups are taken from the known photoelectron spectroscopic data of benzene [ll]. The crossterms B between pairs of individual fly basis orbitals y p , y,, are calculated in the usual fashion from the atomic orbital coefficients of the corresponding linear combinations of y P , y,, using ,8 = 2B,b = - 2.43 eV as a resonance integral between two atomic orbitals connected b y an essential single bond. Finally the symmetry allowed 'through-space' interaction parameters B between the n-orbitals y3, y4, y5 of the left-hand side ortho-phenylene group of 6 rv with the z-orbitals, y6, y7, ?4 , of the right-hand side group have to be adjusted relative to Bbb, = - 0.5 eV used in 1 according to the size of the atomic orbital coefficients of the linear combinations a t the inner centres 11, 12, 13, and 14. Thus no parameters have to be adjusted or new ones introduced to account for the n-orbital interaction in the molecule 6. The following matrix of order 8 is obtained, using the abbreviation B = Bab = - 1.215 eV and B' = Bbb, = -0.5 eV for the cross terms carried over from 1 (all values in eV) :

a),

+

+

+ + + +

+

+

1

2

- 10.0

0

B/@

- 10.0

B/1/2 - 9.25

3

B/1/6

B/1/3

B/vZ

B/1/6

B/1/3

-B/1/6 0

B/1/3 0

B/1/2 B’/2

-B/1/6 0

B/1/3 B’/1/6

0

0

B’/1/c

0

B’/1/6

0

B’/3

0

0

- 9.25

4

-12.25

5 6

- 9.25

symm.

- 9.25

7

0

- 12.25

8

Diagonalization of the above matrix and application of Koopmans’ approximation (- s j = Iv,j)yields (in eV) :

10.69 10.71 12.26 13.20

1 I

10.7 ? ?

The almost perfect agreement between observed (Iv,j(obs)) and calculated ( Iv,j(calc)) vertical ionization energies leaves hardly any doubt that the proposed assignment is the correct one (see Fig. 1).Thus our results fully confirm the original interpretation of the photoelectron spectrum of 1 due to Koenig et aZ. [l]. This work is part no. 97 of project 2.305-0.75 of the Schweizerischer Nationalfonds zur Forderung der Wissenschaften. (Part 96: see ref. [12].) Support by Ciba-Geigy S A . , F . HoffmannL a Roche & Cie. S A . and Sandoz S A . is gratefully acknowledged.

REFERENCES

[l] T . Koenig, R. Wielesek, W . Snell & T . Balle, J. Atner. chem. SOC.97, 3225 (1975). [2] W , Simpson, J . Amer. chem. SOC.75, 597 (1953); W . Simpson & C . Looney, ibid. 76, 6285, 6793 (1954); R . Wielesek, J . Huntington & T . Koenig, Tetrahedron Letters 1974, 2429. [3] D . F . Brailsford & B . Ford, Mol. Physics 18, 621 (1970); J . N . Muvrell & W . Schmid, J. chem. SOC.Faraday 11, 1972, 1709; and references given therein. [4] M . Beez, G. Bieri, H . Bock & E . Heilbronner, Helv. 56, 1028 (1973). [5] E . Heilbronner, R. Gleiter, H . Hopf, V . Hornung & A . de Meijeve, Helv. 54, 783 (1971). [6] F . Brogli, E . Heilbronner & E . Vogel, J . Electron Spectr., in press. [7] R. Hoffmann, Accounts chem. Res..4, 1 (1971). [8] F . Brogli, P . A . Clark, E . Heilbronner & M . Neuenschwandev, Angew. Chem. 85, 414 (1973). [9] J . M . Pearson, H . A . S i x , D.J . Williams & M . Levy, J. Amer. chcm SOC.93, 5034 (1971); D . J . Williams, J . P. Pearson & M . Levy, ibid. 92, 1436 (1970). [lo] J . H . Golden, J . Chem. SOC.1961, 3741. [ l l ] D . W . Turner, C. Baker, A . D . Baker & C . R. Brundle, Molecular Photoelectron Spectroscopy, Wiley-Interscience, London 1970. [lZ] A . J . Ashe III, F . Burger, M . Y . El-Sheik, E . Heibronner, J . P . Maier & J.- F. Muller, Hclv. 59, 1944 (1976).